This invention relates generally to a punch press and, more particularly, to a punch press in which a punch is secured to and is movable with a piston and is advanced to form a hole in a workpiece in response to pressure fluid being admitted into one end of a cylinder which slidably receives the piston. Associated with and movable relative to the punch is a stripper which engages and clamps the workpiece as the punch first forms and then retracts from a hole. The stripper engages the workpiece just before the punch strikes the workpiece, and remains clamped against the workpiece until after the punch has been retracted from the hole. Thereafter, the stripper is released from the workpiece to enable the latter to be re-positioned relative to the punch.
In certain punch presses, the punch is hydraulically retracted and the stripper is hydraulically actuated. That is to say, pressurized oil or other liquid in the cylinder acts against the piston and the stripper for the purposes of (1) retracting the piston and the punch and (2) pressing the stripper into clamping engagement with the workpiece during retraction of the punch. Punch presses of this general type are disclosed in Taylor U.S. Pat. No. 2,716,451 and Brolund et al. U.S. Pat. No. 3,722,337. In the press disclosed in the Taylor patent, the pressurized liquid for retracting the piston and clamping the stripper is not alternately admitted into the cylinder from a pressure source and then dumped from the cylinder to drain but instead is maintained under substantially continuous pressure in a closed system which includes the cylinder. The continuously pressurized liquid thus forms a "hydraulic spring" which effects rapid return of the piston and secures clamping of the stripper.